Contaminated air in buildings and homes is now an international issue. Certain airborne contaminants cause widespread discomfort and health problems, leading to absenteeism from school and work, as well as reduced productivity. Healthy and productive indoor environments would save billions of dollars in health care costs, lost work time, overall output and possible litigation.
Of the many contaminants found in indoor air, bioaerosols are regarded as the leading cause of allergies and other maladies referred to as SBS (Sick Building Syndrome) and BRI (Building-Related Illness). Bioaerosols are airborne products that include microorganisms, their fragments and spores, metabolic gases, and other toxins and waste products. Numerous studies have found high concentrations of these bioaerosols both in air handling equipment and the interiors they serve.
Airborne and surface microorganisms include pathogens, allergens, and toxins. Included in the category of pathogens are viruses, bacteria, and mold, which could cause measles, chicken pox, Legionnaires disease, aspergillosis, tuberculosis, and other infectious disease. Bacteria and mold are also classified as allergens because they can cause allergic rhinitis, asthma, humidifier fever and hypersensitivity pneumonitis. Toxins include mycotoxins and endotoxins, which can cause toxic and allergic reactions, irritations, and odors. Among allergens, mold and mold products are probably the most common worldwide.
Allergy tests universally bear out this phenomenon. In HVAC equipment, mold can proliferate year-round. With most individuals, prolonged exposure to mold and mold products initiates the release of histamines, causing inflammation of mucous membranes, which can be followed by congestion, breathing difficulties, asthma and other respiratory complications.
Conventional HVAC systems are an ideal source and conduit for the origin and/or spread of microorganisms. Their environments are especially conducive to amplifying molds and some bacteria. The fans of the HVAC system disseminate and/or recirculate system, space and occupant-generated microorganisms room to room and person to person. Conventional filtration assemblies of such HVAC systems are compromised because growth typically occurs downstream of filters, which allows microorganisms to seed in the ductwork and travel to and throughout the occupied space. Additionally, viruses and many bacteria are too small to be captured by a common air filter.
Traditional bioaerosol controls tend to be impractical, toxic, detrimental to equipment operation, and costly. Hence, the industry has turned to ultraviolet germicidal irradiation (“UVGI”) for the sterilization of microorganisms. In HVAC systems, the application of UVGI in the air handling unit cooling coil and filter assemblies is effective in reducing the number of microorganisms. Additionally, the constant irradiation exposure has been found to be effective at controlling fungal growth.
Microbes are uniquely vulnerable to the effects of light at wavelengths at or near 2537 Angstroms, due to the resonance of this wavelength with molecular structures. A quantum of energy of ultraviolet light at these wavelengths possesses an amount of energy sufficient to break organic molecular bonds, which damages the cellular structure of the microorganisms.
The ultraviolet component of sunlight is the main reason microbes die in the outdoor air. The die off rate in the outdoors varies from one pathogen to another, but can be anywhere from a few seconds to a few minutes in order to kill 90 to 99% of viruses and contagious bacteria. Spores and some environmental bacteria tend to be resistant and can survive much longer exposures. UVGI systems use much more concentrated levels of ultraviolet energy than are found in sunlight.
The kill rate of pathogens and other microorganisms using UVGI depends on several factors, including UVGI intensity, the number of microorganisms present, and the amount of time the microorganisms are present in the UVGI zone, or dwell time. Generally, kill rate increases as the UVGI intensity is increased and/or the dwell time is increased.